1. Field of the Invention
The present invention relates to a system and method for detecting and correcting phase errors associated with a coherent array.
2. Background Art
Various applications use a coherent array of receivers to detect radiation emitted from a target or scene. The radiation detected by the coherent array can be processed to provide information about the target, such as range information, or to form a two-dimensional or three-dimensional image, for example. Receiver arrays may be used for sensing passive radiation reflected from a target object and for active sensing where a scene or target is illuminated using a known source of radiation, such as a microwave or ultrasound beam. Receiver arrays may be used in diverse applications which employ interferometric, synthetic aperture radar (SAR), or phased-array radar sensors to detect emitted radiation, which applications may include surveillance and reconnaissance, target identification and tracking, or medical applications, for example.
The ultimate accuracy or resolution provided by a particular system employing a coherent array of receivers may be affected by many factors. Phase errors, which may be caused by physical changes of individual receivers in the array, motion of the transmitter(s) and/or receiver array, path propagation delays, electronic propagation delays, etc., may result in undesirable effects, such as blurring. A number of strategies have been developed to detect and/or correct phase errors, many of which depend upon some a priori knowledge or assumptions relative to the target or scene. For example, in correcting phase errors in synthetic-aperture radar (SAR), prominent points in the image may be used to determine the phase errors, as described in W. G. Carrara, R. S. Goodman, and R. M. Majewski, Spotlight Synthetic Aperture Radar Signal Processing Algorithms (Artech House, Boston, 1995), Chapter 6. Another approach for correcting phase errors due to receiver array motion in a interferometric imaging system used in surveillance and reconnaissance uses strain gauges, inertial measurement units, and various focusing algorithms to measure and to compensate for wing flexure and altitude variations of an aerial vehicle as described in U.S. Pat. No. 6,154,174.
A different function is performed by an approach described by R. A. Hutchin in xe2x80x9cSheared Coherent Interferometric Photography: A Technique for Lensless Imagingxe2x80x9d in Digital Image Recovery and Systhesis II, Proc. SPIE 2029, 161-1685 (1993). That approach eliminates the effect of phase errors on a two-dimensional image based on intensity measurements by a receiver array without measuring the phase errors. This is accomplished by using a set of three laser transmitters and detecting at the receiver the interference between signals from pairs of the transmitters.
It is an object of the present invention to provide a system and method for measuring phase errors in multiple-antenna coherent imaging systems.
Another object of the present invention is to provide a system and method for measuring deformation of a surface having an array of receivers.
A further object of the present invention is to provide a system and method for detecting and correcting coherent array aberration which can be applied to interferometric sensors, synthetic aperture radar sensors, and other phased-array coherent imaging systems.
A still further object of the present invention is to provide a non-mechanical means of measuring and correcting phase errors of a coherent receiver array.
Another object of the present invention is to provide a system and method for sensing aberrations of an array of receivers due to unknown motion of the array or propagation delays.
An additional object of the present invention is to provide a method for correcting physical distortions of a receiver array.
In carrying out the above objects and other objects, features, and advantages of the present invention, a system and method for sensing phase errors in a multiple receiver array include transmitting first, second, and third signals from non-collinear transmitters to a target, measuring amplitude and phase of signals reflected from the target corresponding to the first, second, and third signals using the multiple receiver array, computing a first sheared product of the received signals corresponding to the first and second transmitted signals representing information about phase error variations along a first dimension, computing a second sheared product of the received signals corresponding to the first and third transmitted signals representing information about phase error variations along a second dimension, and determining the phase error associated with each receiver in the multiple receiver array based on the sheared products. In one embodiment, phase errors associated with the non-collinear transmitters are measured and subtracted prior to determining the phase error associated with each receiver in the array. In another embodiment, the non-collinear transmitters are mounted such that they all experience the same phase error. The three non-collinear transmitters may be distinguished from the imaging signals received from the receiver array by using three different frequencies or signal modulations outside the imaging bandwidth.
The present invention provides a number of advantages. For example, the present invention provides a light-weight, inexpensive system for measuring and correcting phase errors in multiple antenna coherent imaging systems. The present invention compensates for aberrations caused by a variety of sources including unknown motion of the array, propagation path errors, and/or electronic phase errors. Once sensed using the present invention, aberrations can be subtracted and thereby corrected. Alternatively, aberration information can be used for real-time physical/mechanical correction of a distorted receiver array. For surveillance and reconnaissance applications, the present invention eliminates the need for phase error measurement and associated inertial measurement units at each receiver. Furthermore, the present invention can also be used with interferometric, SAR, and other phased-array coherent imaging sensors with applications in phased-array radars and medical ultrasound, for example.
The above advantages and other advantages, objects, and features of the present invention, will be readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.